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            <div><h3 class="title">
                <a name="asio.reference.Scheduler"></a><a class="link" href="Scheduler.html" title="Scheduler concept">Scheduler
                concept</a>
            </h3></div>
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    <pre class="programlisting"><span class="keyword">template</span><span class="special">&lt;</span><span
            class="keyword">class</span> <span class="identifier">S</span><span class="special">&gt;</span>
  <span class="identifier">concept</span> <span class="identifier">scheduler</span> <span class="special">=</span>
    <span class="identifier">copy_constructible</span><span class="special">&lt;</span><span class="identifier">remove_cvref_t</span><span
                class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;&gt;</span> <span
                class="special">&amp;&amp;</span>
    <span class="identifier">equality_comparable</span><span class="special">&lt;</span><span class="identifier">remove_cvref_t</span><span
                class="special">&lt;</span><span class="identifier">S</span><span class="special">&gt;&gt;</span> <span
                class="special">&amp;&amp;</span>
    <span class="identifier">requires</span><span class="special">(</span><span class="identifier">E</span><span
                class="special">&amp;&amp;</span> <span class="identifier">e</span><span class="special">)</span> <span
                class="special">{</span>
      <span class="identifier">execution</span><span class="special">::</span><span
                class="identifier">schedule</span><span class="special">((</span><span class="identifier">E</span><span
                class="special">&amp;&amp;)</span><span class="identifier">e</span><span class="special">);</span>
    <span class="special">};</span>
</pre>
    <p>
        None of a scheduler's copy constructor, destructor, equality comparison,
        or <code class="computeroutput"><span class="identifier">swap</span></code> operation shall exit
        via an exception.
    </p>
    <p>
        None of these operations, nor a scheduler type's <code class="computeroutput"><span
            class="identifier">schedule</span></code>
        function, or associated query functions shall introduce data races as a result
        of concurrent invocations of those functions from different threads.
    </p>
    <p>
        For any two (possibly const) values <code class="computeroutput"><span class="identifier">x1</span></code>
        and <code class="computeroutput"><span class="identifier">x2</span></code> of some scheduler
        type <code class="computeroutput"><span class="identifier">X</span></code>, <code class="computeroutput"><span
            class="identifier">x1</span>
        <span class="special">==</span> <span class="identifier">x2</span></code>
        shall return <code class="computeroutput"><span class="keyword">true</span></code> only if <code
            class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">query</span><span class="special">(</span><span class="identifier">x1</span><span
            class="special">,</span> <span class="identifier">p</span><span class="special">)</span> <span
            class="special">==</span> <span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">query</span><span class="special">(</span><span class="identifier">x2</span><span
            class="special">,</span>
        <span class="identifier">p</span><span class="special">)</span></code>
        for every property <code class="computeroutput"><span class="identifier">p</span></code> where
        both <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">query</span><span class="special">(</span><span class="identifier">x1</span><span
            class="special">,</span> <span class="identifier">p</span><span class="special">)</span></code> and <code
            class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">query</span><span class="special">(</span><span class="identifier">x2</span><span
            class="special">,</span>
        <span class="identifier">p</span><span class="special">)</span></code>
        are well-formed and result in a non-void type that is <code class="computeroutput"><span class="identifier">EqualityComparable</span></code>
        (C++Std [equalitycomparable]). [<span class="emphasis"><em>Note:</em></span> The above requirements
        imply that <code class="computeroutput"><span class="identifier">x1</span> <span class="special">==</span>
        <span class="identifier">x2</span></code> returns <code class="computeroutput"><span class="keyword">true</span></code>
        if <code class="computeroutput"><span class="identifier">x1</span></code> and <code class="computeroutput"><span
            class="identifier">x2</span></code>
        can be interchanged with identical effects. A scheduler may conceptually
        contain additional properties which are not exposed by a named property type
        that can be observed via <code class="computeroutput"><span class="identifier">asio</span><span class="special">::</span><span
            class="identifier">query</span></code>;
        in this case, it is up to the concrete scheduler implementation to decide
        if these properties affect equality. Returning <code class="computeroutput"><span
            class="keyword">false</span></code>
        does not necessarily imply that the effects are not identical. —<span class="emphasis"><em>end
        note</em></span>]
    </p>
    <p>
        A scheduler type's destructor shall not block pending completion of any receivers
        submitted to the sender objects returned from <code class="computeroutput"><span
            class="identifier">schedule</span></code>.
        [<span class="emphasis"><em>Note:</em></span> The ability to wait for completion of submitted
        function objects may be provided by the execution context that produced the
        scheduler. —<span class="emphasis"><em>end note</em></span>]
    </p>
    <p>
        In addition to the above requirements, type <code class="computeroutput"><span
            class="identifier">S</span></code>
        models <code class="computeroutput"><span class="identifier">scheduler</span></code> only if
        it satisfies the requirements in the Table below.
    </p>
    <p>
        In the Table below,
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                <code class="computeroutput"><span class="identifier">s</span></code> denotes a (possibly
                const) scheduler object of type <code class="computeroutput"><span class="identifier">S</span></code>,
            </li>
            <li class="listitem">
                <code class="computeroutput"><span class="identifier">N</span></code> denotes a type that
                models <code class="computeroutput"><span class="identifier">sender</span></code>, and
            </li>
            <li class="listitem">
                <code class="computeroutput"><span class="identifier">n</span></code> denotes a sender object
                of type <code class="computeroutput"><span class="identifier">N</span></code>
            </li>
        </ul>
    </div>
    <div class="table">
        <a name="asio.reference.Scheduler.t0"></a>
        <p class="title"><b>Table 27. Scheduler requirements</b></p>
        <div class="table-contents">
            <table class="table" summary="Scheduler requirements">
                <colgroup>
                    <col>
                    <col>
                    <col>
                </colgroup>
                <thead>
                <tr>
                    <th>
                        <p>
                            expression
                        </p>
                    </th>
                    <th>
                        <p>
                            return type
                        </p>
                    </th>
                    <th>
                        <p>
                            operation semantics
                        </p>
                    </th>
                </tr>
                </thead>
                <tbody>
                <tr>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span
                                    class="identifier">schedule</span><span class="special">(</span><span
                                    class="identifier">s</span><span class="special">)</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            <code class="computeroutput"><span class="identifier">N</span></code>
                        </p>
                    </td>
                    <td>
                        <p>
                            Evaluates <code class="computeroutput"><span class="identifier">execution</span><span
                                class="special">::</span><span class="identifier">schedule</span><span
                                class="special">(</span><span class="identifier">s</span><span class="special">)</span></code>
                            on the calling thread to create
                            <code class="computeroutput"><span class="identifier">N</span></code>.
                        </p>
                    </td>
                </tr>
                </tbody>
            </table>
        </div>
    </div>
    <br class="table-break">
    <p>
        <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span
                class="identifier">start</span><span class="special">(</span><span class="identifier">o</span><span
                class="special">)</span></code>, where
        <code class="computeroutput"><span class="identifier">o</span></code> is the result of a call
        to <code class="computeroutput"><span class="identifier">execution</span><span class="special">::</span><span
            class="identifier">connect</span><span class="special">(</span><span class="identifier">N</span><span
            class="special">,</span> <span class="identifier">r</span><span class="special">)</span></code> for some
        receiver object <code class="computeroutput"><span class="identifier">r</span></code>,
        is required to eagerly submit <code class="computeroutput"><span class="identifier">r</span></code>
        for execution on an execution agent that <code class="computeroutput"><span class="identifier">s</span></code>
        creates for it. Let <code class="computeroutput"><span class="identifier">rc</span></code> be
        <code class="computeroutput"><span class="identifier">r</span></code> or an object created by
        copy or move construction from <code class="computeroutput"><span class="identifier">r</span></code>.
        The semantic constraints on the <code class="computeroutput"><span class="identifier">sender</span></code>
        <code class="computeroutput"><span class="identifier">N</span></code> returned from a scheduler
        <code class="computeroutput"><span class="identifier">s</span></code>'s <code class="computeroutput"><span
            class="identifier">schedule</span></code>
        function are as follows:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                If <code class="computeroutput"><span class="identifier">rc</span></code>'s <code
                    class="computeroutput"><span class="identifier">set_error</span></code> function is called in
                response
                to a submission error, scheduling error, or other internal error, let
                <code class="computeroutput"><span class="identifier">E</span></code> be an expression that
                refers to that error if <code class="computeroutput"><span class="identifier">set_error</span><span
                    class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span
                    class="identifier">E</span><span class="special">)</span></code> is well-formed; otherwise, let
                <code class="computeroutput"><span class="identifier">E</span></code> be an <code
                    class="computeroutput"><span class="identifier">exception_ptr</span></code>
                that refers to that error. [Note: <code class="computeroutput"><span class="identifier">E</span></code>
                could be the result of calling <code class="computeroutput"><span
                    class="identifier">current_exception</span></code>
                or <code class="computeroutput"><span class="identifier">make_exception_ptr</span></code>.]
                The scheduler calls <code class="computeroutput"><span class="identifier">set_error</span><span
                    class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span
                    class="identifier">E</span><span class="special">)</span></code> on an unspecified weakly-parallel
                execution
                agent ([Note: An invocation of <code class="computeroutput"><span
                    class="identifier">set_error</span></code>
                on a receiver is required to be <code class="computeroutput"><span
                    class="keyword">noexcept</span></code>]),
                and
            </li>
            <li class="listitem">
                If <code class="computeroutput"><span class="identifier">rc</span></code>'s <code
                    class="computeroutput"><span class="identifier">set_error</span></code> function is called in
                response
                to an exception that propagates out of the invocation of <code class="computeroutput"><span
                    class="identifier">set_value</span></code> on <code class="computeroutput"><span class="identifier">rc</span></code>,
                let <code class="computeroutput"><span class="identifier">E</span></code> be <code
                    class="computeroutput"><span class="identifier">make_exception_ptr</span><span
                    class="special">(</span><span class="identifier">receiver_invocation_error</span><span
                    class="special">{})</span></code>
                invoked from within a catch clause that has caught the exception. The
                executor calls <code class="computeroutput"><span class="identifier">set_error</span><span
                    class="special">(</span><span class="identifier">rc</span><span class="special">,</span> <span
                    class="identifier">E</span><span class="special">)</span></code> on an unspecified weakly-parallel
                execution
                agent, and
            </li>
            <li class="listitem">
                A call to <code class="computeroutput"><span class="identifier">set_done</span><span
                    class="special">(</span><span class="identifier">rc</span><span class="special">)</span></code>
                is made on an unspecified weakly-parallel execution agent ([Note: An
                invocation of a receiver's <code class="computeroutput"><span class="identifier">set_done</span></code>
                function is required to be <code class="computeroutput"><span class="keyword">noexcept</span></code>]).
            </li>
        </ul>
    </div>
    <p>
        [<span class="emphasis"><em>Note:</em></span> The senders returned from a scheduler's <code
            class="computeroutput"><span class="identifier">schedule</span></code> function have wide discretion
        when deciding which of the three receiver functions to call upon submission.
        —<span class="emphasis"><em>end note</em></span>]
    </p>
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                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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